Many different species of valve are known, and are used widely in many industries. Many valve designs are operated by some form of user controlled actuator, such as a valve handle, motor, ram or the like. Valves are also known which may be operated in accordance with properties of a fluid under control, such as fluid flow rates and pressures.
Valves are in widespread use in the oil and gas industry. For example, valves are commonly used in downhole injection systems. Examples of such downhole injection systems are provided in WO 2011/157985 and WO 2012/136966, the disclosure of which is incorporated herein by reference.
Oil or gas wells may require fluid to be injected for a variety of requirements. These may include but are not limited to:
Chemical Injection—this may involve the injection of speciality chemicals which are formulated to address issues such as scaling, wax build up, salt built up and the like. Chemical injection applications tend to be performed at very low flow rates which are a very small fraction of the flow rates of the actual produced reservoir well fluids.
Water De-Salting Injection—this may involve the injection of water of either a pure or derived composition to assist in the flushing away of salt deposits in an oil/gas producing region in oil or gas formations. These applications are generally performed at moderate rates of flow which are a small fraction of the flow rates of the actual produced reservoir well fluids.
Diluent Injection—this may involve the injection of a fluid of a special composition for the purpose of reducing viscosity and density of reservoir fluids, for example in order to allow them to be more pumpable to improve or allow production to surface by methods such as a downhole mechanical pump, a downhole electric submersible pump (ESP), gas lift or other such methods of artificial lift. These applications tend to be performed at moderate to high flow rates which are a fraction of the flow rates of the actual produced reservoir well fluids.
Direct Water injection—this may involve the injection of seawater or water recovered from another well into a producing reservoir in order to replenish reservoir pressures and volumes in order to assist in the production from the reservoir. This is generally performed at very high rates of flow comparable to the production flow rates that may occur form the reservoir.
In all instances of the above example fluid injection applications the line carrying the fluid to be injected must be equipped with a non-return or check valve, such that flow in only one direction is permitted. This is necessary to ensure that any fluid pressures encountered specifically in the reservoir at the point of injection shall be stopped from reverse flow to surface as a means of protecting surface equipment and facilities from the risk of reservoir fluids being delivered to these surface locations.
A check valve can take many forms. Generally, check valves include a moveable valve body or structure which cooperates with a valve seat. The valve body is lifted from the valve seat in response to flow or sufficient pressure in a forward direction, and moved and held against the valve seat in response to flow or sufficient pressure in the reverse direction. In most cases the valve body is biased towards a closed position, such that the valve body will only open when pressure in a forward direction exceeds the effect of the bias. The minimum pressure required to open the valve body is typically referred to as the cracking pressure.
The most basic is the ball or poppet design where a ball or pin (poppet) is moved to a positively closed position by the force of a spring to stop reverse flow of fluids from an outlet to inlet. In order to allow flow in the forward direction (from inlet to outlet), the inlet pressure must be pressurised to a high enough level to overcome the force of the closure spring to open the ball or pin, which will then allow fluid to forward flow.
The ball and poppet check is ideally suited to lower flow regimes such as may be found in, for example, direct chemical injection. For higher flow rates, ball and poppet checks suffer the disadvantage of having an obstruction to the direction of flow. This can lead to erosion and wear of the sealing components of the check which can then lead to the reverse flow sealing capability of the check being compromised. Also, the projection of the internal components is directly in the flow path of the fluid passing through the device which can then lead to increased pressure losses. Therefore ball and poppet checks are typically not suited to higher flow regimes.
For higher rates of flow, other forms of check tend to be used. For high flow regimes forms such as butterfly checks are used. These operate by way of one or two plates which are hinged to allow rotation into an open position to provide a large flow path for fluid passing through. The plates are returned to their closed position by springs. However, although such butterfly checks might offer the advantage of a higher flow area, they are not always suited to downhole sealing requirements due to the complex shape of the sealing plates. As such, butterfly checks are normally confined to surface applications.
An alternative but similar approach is the flapper type of check which operates by way of fluid flow moving the swing plate (flapper) about a fixed rotating axis against a closure spring. This then allows a larger flow area and reduced fluid pressure losses through the device.
For downhole applications these example forms of device generally require modification in order to be fully suitable for use in an oil/gas well environment due to aggressive fluids, elevated temperatures and the need for a long service life capability in providing a critical reverse flow protection from reservoir fluids. For low flow applications such as direct chemical injection or water de-salting ball or poppet checks may be suitable. However, for requirements where higher flow rates are required such as higher flow rate water de-salting, diluent injection and direct water injection, devices based on flapper or articulated ball devices may be preferred. However, such devices also have their limitations, such as their required size, exposure of seal surfaces to the high flow rates when opened, and the like.
Also, many known check valves are sensitive to varying flow rate situations, and may suffer problems in such varying flow rate situations. Ball and poppet checks will have a range of flow where the ball or poppet is trying to float in a partially open position. Because the ball or poppet is driven to a closure position by a return spring which opposes the path of flow, the ball or poppet can be unstable and oscillate back and forth onto its sealing region causing damage to the critical sealing area of the device.
With a swing or butterfly check a similar mode can occur where the plates of the check are partially open and exposed to the flow path and also are subject to oscillation which may damage their function as a non-return barrier protection.
Current systems may therefore not be suitable for fluid injection applications where variable flow rate requirements must be catered for while still assuring the reverse flow protective barrier is suitably protected and will operate in arduous downhole conditions for a long life span.